1. Field of the Invention
The present invention relates to an image formation apparatus that forms color images, such as a copying machine, a printer, a facsimile, a plotter, or an ink jet recorder, an alignment pattern detection sensor as an alignment pattern detection unit used for the image formation apparatus, a color misalignment detection method, and a color misalignment correction method.
2. Description of the Related Art
Conventionally, a predominant color image formation apparatus has been a type in which one photosensitive drum and a revolver type development apparatus are used to form toner images of respective colors, the respective toner images are transferred onto an intermediate transfer body by superposition, and then collectively transferred onto transfer paper as a sheet-form recording medium.
On the other hand, with the recent tendency of speed-up and high performance of a color image output device, a so-called four-drum tandem type color image formation apparatus has become predominant. This four-drum tandem type color image formation apparatus has a configuration such that a plurality of image formation units each including a photoreceptor (image carrier) and a development apparatus corresponding thereto are disposed for each color at positions facing a transfer belt, and that toner images on the image carriers are sequentially transferred onto transfer paper or a transfer belt.
In such a type of color image formation apparatus, the toner images formed on the image carriers for respective colors can be transferred at the same time, and therefore there is an advantage in that the printing speed can be increased, but there is a disadvantage with respect to color misalignment between the respective colors due to the method, as compared to the conventional one-drum intermediate transfer type color image formation apparatus.
As this type of technique, inventions disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-81745 and Japanese Patent Application Laid-Open No. 2001-249513 are well known. These publications disclose a misalignment pattern and a detection method thereof in the four-drum tandem type color image formation apparatus.
With regard to the technical problem of the color misalignment, many correction methods are heretofore proposed. For example, in Japanese Patent Application Publication No. HEI 7-19084, a technique is disclosed in which a line image in each color is formed on a transfer belt, the passage of the line image is detected by a detection sensor, and a displacement quantity of the line image in each color from an ideal passage timing is measured, to thereby obtain a misalignment quantity in each color and perform correction.
Since such a technique is a method for detecting an edge of a pattern passing the detection sensor, the detection accuracy is determined by the sampling frequency. In other words, if a machine has a resolution of 600 dpi, and the unit of correction is 42.3 μm (=25.4/600×1000), at least the detection of ±½ (=21.7 μm) or less of the unit of correction is demanded. When the linear velocity of the line image on the transfer belt is 125 mm/sec, a minimum required sampling frequency is calculated as at least 6 kHz, from an expression of [sampling frequency]=[linear velocity]/[25.4/resolution dpi/2], but the detection accuracy (=detection error) in this case (=6 kHz) becomes 21.7 μm.
If this figure is directly fed back to the misalignment correction, there may be no problem with such a degree of sampling frequency. However, there is a case where it is necessary to use this detection result (=x μm) for other calculation. For example, when such detection is performed at left and right opposite ends with respect to the paper transport direction and skew correction is carried out or magnification error correction is carried out, based on the detection results at the opposite ends, higher detection accuracy is required. Therefore, for example, when 2 μm is required as the detection accuracy, it is necessary to increase the sampling frequency as high as 60 kHz.
As described above, since the necessary sampling frequency is in proportion to the linear velocity and resolution, high processing speed that corresponds to the high-speed sampling becomes necessary for processing blocks performed after the data sampling. Hence, there is a problem in that the cost required for color misalignment correction increases in proportion to the increase in the speed of apparatus.
As detection means for improving the detection accuracy of the pattern edge, a method of detecting the pattern edge by a Charge Coupled Device (CCD) sensor having high accuracy and high resolution is proposed, but even when such detection means is used, problems such as complication and cost increase of the apparatus cannot be avoided.
In order to deal with these problems, for example, in Japanese Patent No. 3254244, a technique as follows is disclosed. Each average density of a toner image pattern formed by superposing a second color toner image on a first color toner image and of a toner image pattern formed by shifting the relative position of the two color patterns by a predetermined quantity, is detected by an optical sensor, and the misalignment quantity between the first color and the second color and the direction of the misalignment are determined from the signal output of the optical sensor, to correct the misalignment.
In this technique, the detection of the misalignment quantity is performed not by detecting the edge of the pattern image (line image), but by detecting an averaged output signal of the optical sensor detected from the whole pattern. Therefore, it is possible to detect the misalignment quantity by a sampling frequency as low as 500 Hz or below (for every 2 msec), that is, about 1/100 as compared with the one disclosed in Japanese Patent Application Publication No. HEI 7-19084.
Therefore, when the misalignment detection method disclosed in Japanese Patent No. 3254244 is used, the hardware can be configured at a lower cost relating to the misalignment quantity detection if a detection accuracy of the same level as with the technique disclosed in Japanese Patent Application Publication No. HEI 7-19084 can be obtained, and hence considerable cost reduction becomes possible.
The technique similar to the misalignment detection method described in Japanese Patent No. 3254244 includes techniques disclosed, for example, in Japanese Patent Application Laid-Open (“JPA”) No. HEI 10-329381, JPA No. 2000-81745, JPA No. 2001-209223, JPA No. 2002-40746, and JPA No. 2002-229280.
If it is considered to perform misalignment correction based on the output signal from the optical sensor with respect to a superposed pattern of two color toner images, described in Japanese Patent No. 3254244, and if it is assumed that the maximum correction quantity to be corrected is ±10 dots, then the misalignment correction quantity and the direction thereof can be determined by forming 21 patterns obtained through shifting the relative position of the two colors dot by dot and reading extreme values of the patterns.
However, by creating patterns as many as 21, not only wasteful toner consumption increases, but also the time required for automatic misalignment adjustment increases, which is not desirable.
Relating to this problem, for example, JPA No. HEI 10-329381 discloses a method for detecting misalignment quantity more accurately, by calculating an intersection point of two lines, when the reflected optical density is plotted on the longitudinal axis, with respect to a printing position parameter plotted on the lateral axis.
With respect to the technical problem of the color misalignment, various color misalignment correction methods are heretofore proposed. One of these is a correction method in which a plurality of line images of the respective colors is formed on a transfer belt, and the color misalignment is corrected from the absolute position of the line image. When such a method that the color misalignment quantity of the respective color line images with respect to a reference color line is detected to correct this is chosen, a method in which a line edge is detected from an output of reflected light of the light irradiated to the line is taken as a specific method. In this method, however, the sampling frequency must be increased (corresponding to speed-up of the apparatus), in order to improve the edge detection accuracy, and in addition to this, high processing speed becomes necessary. Therefore, there is a problem in that the cost required for color misalignment correction increases in proportion to speed-up of the apparatus.
Particularly, in order to improve the edge detection accuracy, a method for detecting the edge by a CCD sensor having high accuracy and high resolution is proposed. However, even when such a method is used, there are technical problems such as complication and cost increase of the apparatus.
Therefore, Japanese Patent Application Laid-Open No. 2001-249513 discloses a method in which after a reference color having different pattern pitch and a measurement color to be corrected are formed by superposition, a change in quantity of light corresponding to one cycle of the superposed color pattern is detected, without detecting the line edge, and color misalignment between these is detected based on the detection information and corrected. When such a method is used, a displacement in the direction of reading the line (that is, a displacement in the vertical scanning direction) and a displacement in the horizontal scanning direction (that is, skew) can be detected, but it is considered that correction with respect to the displacement in the horizontal scanning direction is difficult, and no specific method relating to this is specified.
On the other hand, Japanese Patent Application Laid-Open No. 2000-81745 discloses a method in which a reference color and a color to be corrected in a pattern comprising a plurality of lines having the same width and a line interval equal thereto are superposed, and a density detection value of the superposed pattern is compared with a density value D0 under an ideal condition when the pattern images are exactly superposed on each other, to thereby correct the color misalignment.
According to this method, it is indicated that the displacement quantity in the horizontal scanning direction and the vertical scanning direction can be detected by forming such a single patch. Actually, however, the difference between the reference density D0 and the detected value largely changes due to the toner density of the respective colors, the light emission current of an Light Emitting Diode (LED), being a detection sensor, and the detection distance of the sensor (distance from an object to be measured to the sensor), and even if the pattern is formed only by the reference color, in order to correct the value of density D0 of the reference pattern (a pattern formed by superposing the reference color and the color to be corrected) by the toner density value at that time, since the total thickness of the toner is different between this pattern and the reference pattern density D0, the both do not become equal, thereby causing a detection error in the color misalignment correction quantity.
According to the method disclosed in Japanese Patent Application Laid-Open No. HEI 10-329381, even if the maximum correction quantity is ±10 dots, it is not necessary to form 21 patterns, and 11 patterns shifted by several dots appropriately, for example by 2 dots, need only be formed. If the patterns are shifted by 5 dots, then only five patterns are required. As a result, highly accurate misalignment correction can be realized, while considerably reducing the number of patterns and the time required for misalignment adjustment.
Since misalignment adjustment is an operation that has no relation to the normal printing operation, if the processing time is long, the time required for the first print increases. Therefore, the shorter such adjustment time, the better, in view of the productivity.
However, when the misalignment quantity is obtained by calculating an intersection point of a linear approximate expression of two lines, such an output characteristic must be obtained that a sensor output signal of each patch linearly increases or decreases with respect to a predetermined optional shift quantity, that is, a line in which a coefficient of determination R2 in each approximate expression of two lines is infinitely close to 1 must be obtained.
Therefore, for example in a color image formation apparatus of four-drum tandem direct transfer method (a method in which transfer paper is electrostatically attracted onto a transfer belt 18, and images in the respective colors are sequentially transferred onto the transfer paper and superposed) as shown in FIG. 1, an experiment is carried out in the following manner. That is, as shown in FIG. 45, each color line in a patch configured by superposing two color lines of black (Bk), being a reference color, and another color (for example, cyan (c)) is formed by superposition of one line, being the minimum number, is designated as one patch. A detection pattern (alignment pattern) Pk for misalignment detection in the horizontal scanning direction obtained by continuously forming 13 patches (P1 to P13) with the relative position of the two colors shifted by an optional quantity is read by a conventional optical sensor (alignment pattern detection sensor) as shown in FIGS. 46A and 46B, and an output voltage of each patch with respect to the optional shift quantity of the line other than the reference color is plotted.
In FIG. 45, the respective patches are arranged along the scanning direction of the optical sensor, that is, the movement direction of the transfer belt, and the color other than the reference color is shifted by an optional quantity, in a direction orthogonal to the direction, in order to detect color misalignment in the horizontal scanning direction.
In FIGS. 46A and 46B, the optical sensor comprises a Light Emitting Diode (LED) 700, a regular reflected light photoreceptor 701, and a diffused light (hereinafter also referred to as diffused reflected light) photoreceptor 702, and these elements are supported on a support base 703. These elements are actually arranged in a substantially vertical plane with respect to a moving plane of an alignment pattern, but in FIG. 46A, it is displayed on a plane by bringing it down by 90 degrees for easy understanding. In FIG. 46B, reference sign 700a denotes a spot shape of the LED 700, and reference sign 701a denotes a spot shape of the regular reflected light photoreceptor 701, and reference sign 702a denotes a spot shape of the diffused light photoreceptor 702.
As a result of experiments, as shown in FIG. 47, in the approximate line obtained by plotted points on the negative side with respect to the extreme value, R2 is 0.9275. On the other hand, in the approximate line obtained by plotted points on the positive side with respect to the extreme value, R2 is 0.9555. Thus, an output characteristic that cannot be said as a straight line has been obtained.
As a result of calculation of an intersection point by the two approximate lines, the misalignment quantity of 34.74 μm (=0.82 dot) is obtained. The experiment conditions of the detection pattern and the detection sensor are as follows.
Detection Pattern: (=detailed parameter of a pattern shown in FIG. 45)
Bk line width: 24 dots (=1.016 mm)
Color line width: 24 dots (=1.016 mm)
Optional shift quantity: 4 dots (=25.4/600×1000×4=169.3 μm)
Total number of patches: 13 patches (in P1 and P13, the both are not superposed completely, and in P7, the both are completely superposed)
Detection Sensor: (=detailed specification of a sensor shown in FIGS. 46A and 46B)
Element on light emission side: GaAs infrared light emission diode (peak emission wavelength, λS=950 nm); top view type spot diameter: 1.0 mm; element on photoreceptor side: Si phototransistor (peak spectral sensitivity, λS=800 nm); top view type spot diameter: regular reflected light receiving side, 1.0 mm; diffused reflected light receiving side, 3.0 mm; Detection distance: 5 mm (distance from upper part of the sensor to an object surface to be detected (patch))
Linear Velocity:
245 mm/sec [sampling frequency]
500 Sampling/sec